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Swelling-Activated Cl^– Channels Support Cl^– Secretion by Bovine Ciliary Epithelium

¹From the Departments of Physiology and ³Medicine, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania; and ²School of Optometry, The Hong Kong Polytechnic University, Kowloon, Hong Kong.

PURPOSE. To determine whether swelling-activated Cl^– currents (I_Cl,swell) observed in isolated nonpigmented ciliary epithelial (NPE) cells contribute to Cl^– secretion across the ciliary epithelium.

METHODS. Ion transport across intact bovine ciliary epithelium was monitored electrically. Native isolated bovine NPE cells were harvested enzymatically. Cell volume changes were measured by calcein-fluorescence quenching.

RESULTS. Bilateral reduction in osmolality transiently increased short-circuit current (I_sc), averaging 60% to 70%. Bilateral pretreatment with 5-nitro-2-(phenylpropylamino)-benzoate (NPPB), a Cl^– channel blocker, reduced I_sc stimulation by 60%, suggesting that transcellular I_Cl,swell largely mediates the increased current. The hypotonically-triggered I_sc stimulation was also inhibited by phloretin, a blocker of swelling-activated Cl^– channels and by flufenamic acid, a blocker of Cl^– and nonselective cation channels. Cyclamate substitution for bath Cl^– reduced the baseline I_sc and the increase in hypotonically-triggered I_sc. In that case, addition of either NPPB or flufenamic acid did not produce further inhibition. The transepithelial responses were correlated with regulatory volume responses of freshly harvested NPE cells. Hypotonicity elicited a regulatory volume decrease (RVD) over a period comparable to that of the hypotonicity-triggered increase in I_sc. The RVD was also inhibited by Cl^–-channel blockers and by Cl^– substitution.

CONCLUSIONS. I_Cl,swell of NPE cells is functionally expressed in intact ciliary epithelium and is oriented to subserve aqueous humor formation. NPE cell volume can be measured with calcein-fluorescence quenching. I_Cl,swell may be stimulated by increased stromal fluid uptake and delivery to the NPE cells, facilitating Cl^– secretion and increasing fluid release into the posterior chamber.

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